Electroluminescent polymers have been extensively studied in the past few years owing to their promising applications in polymeric light-emitting devices (PLEDs), photovoltaic diodes, field emission transistors (FETs) and lasers. In the last decade, various types of conjugated polymers for use in light-emitting devices have been described. Fully conjugated polymers and their derivatives such as poly(p-phenylenevinylene) (PPV) (Burroughes, et al., Nature, 1990, 347, 539), poly(p-phenylene) (PPP) (Grem, et al., Adv. Mater., 1992, 4, 36), poly(fluorene)s (PF) (Fukuka et al., J. Polym. Sci. Polym. Chem., 1993, 31, 2456), and poly(3-alkylthiophene)s (Berggren, et al., Nature, 1994, 372, 444), poly(thienylvinylene) (Lowe et al., Can J. Chem (1998) 76: 1524-1529) amongst others, have been used.
Poly(p-phenylenevinylene) (PPV) and its derivatives have been widely used as emissive materials for light-emitting devices. Significant progress in improved efficiency, decreasing driving voltage and prolonging the lifetime of the devices have been made using PPVs as the emissive layers for LEDs in the past decade. In particular, applications in PLEDs have attracted significant attention following the first report of the PLED device based on poly(p-phenylenevinylene) (PPV) in 1990, mainly because of their fascinating perspectives of low cost processing through simple spin-coating or casting techniques, colour emission in the full range of visible spectrum, low driving voltage and large area display, and flexible structures.
The performance (e.g. efficiency and lifetime) of PLED devices depends on a large variety of factors, such as the device architecture, encapsulation method, preparation method, driving scheme, and the polymer itself. From the materials point of view, high performance devices require electroluminescent polymers with tailored properties such as high photoluminescence (PL) quantum efficiency, good processability, and high thermal/optical/electrical stability. So far, numerous conjugated polymers have been successfully synthesized and used as the active layers and/or transporting layers in PLEDs such as, PPV and its derivatives, PPP, polythiophenes (PTs), PF, poly(thienylenevinylene) (PTV) as well as their copolymers. Among them, PPV and their alkoxy derivatives are the most commonly used materials for PLEDs on account of their good device performance. In general, there are two approaches adopted for the preparation of processable PPVs—(i) the precursor approach and (ii) the side chain approach.
Currently, several precursor routes, namely the sulfonium, halogen and xanthate precursor routes, are used for the fabrication of PPV LEDs (WO93/14177). The precursor polymers, which are soluble in water or organic solvents, are converted to the fully conjugated form through a heat treatment process often at temperatures exceeding 200° C. Although the precursor polymer route may have some advantages, the multi-step synthesis is complex and the resultant polymers may contain structural defects. In addition, as a result of the heat treatment, the precursor route may not be applied to produce flexible plastic displays. These disadvantages limit its application.
The side chain approach involves the polymerization of suitably substituted monomers to form a soluble PPV derivative that can be directly cast into thin films. U.S. Pat. No. 5,189,136 discloses a partially soluble PPV polymer of poly(2 methoxy-5-(2′-ethylhexyloxy)-para-phenylenevinylene) (MEH-PPV) which may be processed into shaped articles, films, fibers, and the like. However, MEH-PPV is only partially soluble and MEH-PPV solutions are unstable and form gels at room temperature. Casting of uniform films of MEH-PPV is quite difficult and requires high experimental skill such that it is not economical.
Other soluble alkoxy substituted PPVs include poly(2,5-bis(2′-ethylhexyloxy)-p-phenylenevinylene) (BEH-PPV) and poly(2,5-bischolestanoxy-p-phenylenevinylene) (BCHA-PPV), which have demonstrated orange and yellow light emission with medium efficiencies in LED devices (U.S. Pat. No. 5,679,757).
Besides alkoxy substituted PPVs, another type of PPV polymer is aryl-substituted PPVs. Aryl groups can be attached at the phenylene rings and/or at the vinylene bridges of PPV. Both approaches may enhance the solubility of the resulting polymers. In addition to improved processability, aryl substitution may also enable enhanced photoluminescence efficiency and photo-stability. PPV normally emits green-yellow light and pure green light emission may be realized from aryl-substituted PPVs, due to the enlarged band gap associated with the steric effects of aryl side chain on the PPV backbone. Examples of aryl-substituted PPVs are poly(2,3-diphenyl-p-phenylene) (B. R. Hsieh, et al. (1995) Adv. Mater. 7: 36), poly(2-(2′-ethyl)hexyloxy-5-(10′-phenyl)anthryl-9′-yl-p-phenylene vinylene) (S. J. Chung, et al. (1998), Adv. Mater. 10: 684), and a series of phenyl-substituted PPV-based copolymers disclosed by Covion Organic Semiconductors (H. Spreitzer, et al, WO98/27136). These polymers have demonstrated good to excellent performance relating to quantum, power and luminance efficiencies. However, PPV derivatives prepared via the Gilch polymerization route may contain tolane-bisbenzyl (TBB) structural defects in the polymer backbone, which are thought to result from head-to-head or tail-to-tail polymerization instead of regular head-to-tail polymerization. The TBB structural defects in phenyl-substituted PPVs are even more severe than the standard dialkoxy PPVs such as MEH-PPV and OC1C10-PPV (5-6% vs. 1.5-2.2%). It was also shown that the operational lifetime was dramatically affected by the amount of TBB.
WO98/27136 discloses a new aryl substituted PPV based light emitting polymer called Super Yellow in which a strong electron donating methoxy group is attached to the PPV backbone to guide the polymer chain propagation during polymerization, which can partially suppress the formation of TBB structural defects. Super Yellow has demonstrated quite promising performance in terms of device efficiency and lifetime. However, this polymer is a yellow emitter, which is not very desirable for full color display applications.
When a biphenyl group is attached to PPV polymer backbone through position-2 of the side group, the resulting polymer can show as little as 0.36% TBB structural defects and the polymer emits green light (Chen Z K et al, Macromolecules, 2003, 36(4), 1009). This polymer, however, showed a tendency to form gels during storage and the quantum efficiency of the polymer is not satisfactory. Therefore, there is a need to develop stable and efficient light emitting polymers which can be easily solution processed.